A reverse osmosis (RO) separation process is becoming increasingly important in the last two decades for desalination and waste water reuse process. An RO process is aimed at replacing known energy inefficient processes such as thermal processes & chemical processes. This was possible after (i) discovering low pressure and high rejection membrane at lower cost; and (ii) integration of an energy recovery device (ERD) for recovering energy from reject stream. Even though membrane separation processes can be more energy efficient than other known processes, the membrane separation can involve membrane fouling (deposition of unwanted material at membrane surface due to concentration polarization), such as the accumulation of solute molecules near the membrane surface. These materials can be either biological or chemical in nature resulting in bio-fouling or chemical fouling, respectively. As a result, a membrane separation process can become inefficient over a period of operation due to the decrease in production flow rate because of continuous fouling at membrane surface.
In general the RO trains are operated at a recommended feed pressure and feed flow rate given by membrane operators, but in real system the optimal operating condition such as feed pressure & flow rate will be changing with respect to the membrane operating condition. To address this issue, a Membrane Performance tool is available to estimate optimal operating conditions for controlling membrane fouling condition.
Although the membranes can be operated at their optimal operating conditions, the fouling at a membrane surface cannot be avoided in the membrane separation process. Therefore the chemical cleaning and membrane replacement are inherent activities for membrane separation processes. Presently the schedule for membrane replacement and chemical cleaning can be suggested by either a membrane expert (consultant) or membrane manufacturers.
In order to reduce membrane fouling in an RO process, the feed water to a membrane can be treated continuously with fouling control chemicals such as antiscalants. Though antiscalants dissolve the substances accumulated near the membrane surface and reduce the rate of fouling, the high dosage of antiscalants can lead to an increase in RO membrane degradation. Therefore, controlled addition of antiscalants can be used for controlled membrane fouling, leading to minimal membrane degradation and lower chemical consumption.
Also membrane cleaning chemicals can be added to clean the membrane at a certain flow rate and concentration as suggested by the membrane manufacturer. Optimizing the chemical flow rate and cleaning cycle based on the current membrane fouling state will reduce the cost of chemicals. This will also help in reducing the production off time due to cleaning, and hence increase the RO permeate production.
The membrane fouling/cleaning is poorly understood due to lack of physical understanding about the interaction between (i) fouling material and membrane, (ii) among fouling materials, (iii) fouling material and cleaning chemicals, and (iv) between membrane and cleaning chemicals. Since the cleaning phenomena is not well understood, an existing practice is to keep a membrane soaked in cleaning chemicals for a fixed period of time and then bring it back to operation.
A current practice of membrane cleaning is based on recommendations from membrane manufacturers which may consume more cleaning chemicals since the recommendations are given based on feed water quality, and are not based on severity of fouling. In addition, these chemicals are also quite costly. Membrane cleaning processes are not well automated and there is an opportunity to develop an advanced tool for estimating effectiveness of the membrane cleaning and optimizing of the cleaning operation.
The membrane cleaning chemistry and its hydrodynamics can be vital in assessing the effectiveness of membrane cleaning. However, it can be very difficult to represent these processes mathematically due to their complex nature. Therefore, a simple alternative method to identify the effectiveness of the membrane cleaning would be helpful.
By estimating the effectiveness of the membrane cleaning, the cleaning chemical consumption and the plant down time can be minimized.
As can be inferred from the above discussion, the membrane cleaning process can be time consuming and not optimal due to excessive use of chemicals. Accordingly, a method and system for optimization of a membrane cleaning process are disclosed.